In order to improve the thermodynamic properties of internal combustion engines, valve trains are known, of which the operating cycle can be influenced in order to make it possible, for example, to vary, as a function of the rotational speed, the opening times or the lift of the gas exchange valves.
DE 10 2004 011 586 A1 of the applicant already discloses a valve train of the type that is described in the introduction. In this case, a plurality of cam carriers having a complementary internal toothing are arranged in a rotationally fixed and axially movable manner on a base camshaft, which is provided with an external toothing. In order to actuate two gas exchange valves of a cylinder, the associated cam carrier has two cam profile groups, each of which is arranged at an axial distance from each other and each of which has two different cam profiles. When the cam carriers are displaced axially on the base camshaft between two defined displacement positions, one of the two cam profiles of each cam profile group can be moved into abutting contact with a roller of a roller cam follower of the respective gas exchange valve. In order to hold the cam carriers in the two displacement positions in defined axial positions, the prior art valve train has stopping devices, each of which comprises a pressure-applying element in the form of a locking ball, which can be inserted into a radial blind borehole of the base camshaft and is pressed radially outward against an opposite inclined flank of a locking channel or locking groove by means of the force of a helical compression spring in the borehole. As a result, the locking balls act on the respective cam carrier with a radial and an axial force component, of which the latter serves to push the cam carrier against a front surface of a bearing block that serves as a stop and, in so doing, to hold this cam carrier in a defined axial position.
In the case of the valve train known from the prior art, the stopping devices are arranged radially inward from a cam profile group in close proximity to one of the two front ends of each cam carrier. In conjunction with the radial clearance, required to displace the cam carrier, between the external toothing of the base camshaft and the internal toothing of the cam carrier, the result is a slightly inclined position of the cam carrier. This in turn leads, upon each actuation of the valve, to an audible noise, when at the opposite front end of the cam carrier that is not pressed against the base camshaft, the internal toothing of the cam carrier strikes against the external toothing of the base camshaft owing to a change in engagement in the vicinity of the maximum valve lift.
Another factor that causes the generation of noise in the valve train is that in order to facilitate the production of the base camshaft, all of the blind boreholes, which serve to accommodate the helical compression springs and the stop balls, are aligned parallel to each other and terminate on the same side of the base camshaft. However, the different opening times of the gas exchange valves of the adjacent cylinders and the resulting necessary angular offset of the lift curves of the cams on the adjacent cam carriers may result in the radial forces, exerted on the cam carriers by the stop balls and/or the pressure-applying elements, having a different angular orientation with respect to the lift curves of the cam carriers, a state that may also be the cause for noises. Moreover, the parallel alignment of all boreholes also has the drawback that the reaction forces, exerted on the base camshaft by the helical compression springs, have altogether the same direction, so that the base camshaft is supported unilaterally via the cam carriers and is bent in this direction.
Working on this basis, the object of the invention is to counteract noise generation in the valve train.